(1) Field of the Invention
The present invention relates to an improved metal-deposited laminated film for a capacitor and a method for making a capacitor element by using this improved laminated film.
(2) Description of the Related Art
With the recent reductions in the size and weight of electric appliances, the size of capacitors used therein has been gradually reduced, and thus, a reduction in the thickness of the films used for the capacitors has become necessary. The thickness of a thinnest polyethylene terephthalate film now available is 1.5 .mu.m, but this film is very difficult to handle because of the extreme thinness of the film. If a film having a further reduced thickness is developed, this film will be very effective for increasing the capacitance or reducing the size of a capacitor. However, if the film thickness is thus reduced, the stiffness of the film is drastically lowered and such operations as vacuum deposition on the film, slitting, element winding, coating, and lamination become very difficult. Thus it has been desired to develop a method by which capacitor elements having an ultra-thin dielectric layer can be advantageously made from an ultra-thin film.
As means for improving the poor operation adaptability of an ultra-thin film, there is known a method in which the ultra-thin film is laminated onto a different film to form a thick laminated film and, after processing, it is peeled off from this laminated film. For example, Japanese Unexamined Patent Publication No. 58-5226 discloses a method in which a laminated sheet is prepared by co-extrusion of a polyolefin and polyethylene terephthalate, the laminated sheet is drawn, and the polyethylene terephthalate is then separated from the laminated sheet to obtain an ultra-thin film. This publication also suggests that a metal may be vacuum-deposited on this ultra-thin film.
There are many methods known in which different polymers are laminated, the laminate is drawn, and one polymer film is separated from the laminate to form an ultra-thin film. These known methods are disclosed in, for example, Japanese Unexamined Patent Publications No. 58-132520, No. 58-136417, No. 57-176125, and No. 52-37982. A defect of these known methods is that it is very difficult to obtain a long, unbroken ultra-thin film by continuously separating the ultra-thin film from a substrate film. In other words, the ultra-thin film is usually cut or broken during the separation. This is a serious obstacle to the practical working of these methods.